1. Field of the Invention
The present invention relates to a valve gear for an engine that switches between a mode in which an intake valve or exhaust valve normally operates, and a mode in which the operation of the intake valve or exhaust valve stops.
2. Description of the Related Art
Conventionally, a technique for resting some cylinders while an engine is operating has been known as a technique to further improve fuel consumption. Resting some cylinders is often performed by assembling a switch into a valve gear that drives an intake valve or an exhaust valve.
A conventional switch of this kind is described in, e.g., Japanese Patent Laid-Open No. 2008-151115.
The switch described in Japanese Patent Laid-Open No. 2008-151115 has a structure which switches the support modes of a rocker arm that drives an intake valve or an exhaust valve. The support mode of the rocker arm is switched to a normal operation state support mode or a cylinder resting state support mode.
In the normal operation state, the switch converts the rotation of a cam shaft into a reciprocal motion by using the rocker arm, and transmits the reciprocal motion to the intake valve or the exhaust valve. On the other hand, in the cylinder resting state, the position of the swinging center of the rocker arm changes, i.e., the rocker arm swings around one end that is in contact with the intake valve or exhaust valve, against the spring force of a return spring which biases the other end of the rocker arm. That is, only the rocker arm swings with the intake valve or exhaust valve being closed.
The position of the swinging center of the rocker arm is changed by using a plurality of rocker shafts. The switch includes a first rocker shaft that functions as the rocker arm swinging center in the cylinder resting state, and a second rocker shaft that functions as the rocker arm swinging center in the normal operation state. The first and second rocker shafts are connected with each other by a connecting rod, and are integrated into one rocker shaft assembly. The rocker shaft assembly is able to move in the axial direction between the rocker arm and rocker arm support members provided on the two sides of the rocker arm. Also, the rocker shaft assembly is moved to one side or the other side in the axial direction during switching by being driven by an actuator.
The first and second rocker shafts are parallel and spaced apart from each other in a direction perpendicular to the axial direction, and divided into one side and the other side of the connecting rod in the axial direction. The first and second rocker shafts are provided in shaft holes of the rocker arm support members so as to be pivotal and movable in the axial direction. The connecting rod is accommodated in a groove in the rocker arm support member.
In the normal operation state, the rocker arm assembly moves toward the rocker arm support member. In this state, the first rocker shaft is detached from the rocker arm and accommodated together with the connecting rod in the rocker arm support member. The second rocker shaft pivotally fits into both the rocker arm support member and the rocker arm. In the normal operation state, therefore, the rocker arm swings around the second rocker shaft.
In the cylinder resting state, the second rocker shaft and the connecting rod extend out from the rocker arm support member, and the first rocker shaft pivotally fits into both the rocker arm support member and the rocker arm. In the cylinder resting state, therefore, the rocker arm swings around one end of the first rocker shaft that is in contact with the intake valve or exhaust valve.
The valve gear for an engine described in Japanese Patent Laid-Open No. 2008-151115 has a problem that the reliability of the operation of switching the rocker arm support modes is low. This problem occurs due to the following two reasons.
The first reason is that the structure of the rocker shaft that supports the rocker arm to be swingable is complicated. When the normal operation state support mode shifts to the cylinder resting state support mode, the second rocker shaft and the connecting rod must be detached from the rocker arm support member. On the other hand, when the cylinder resting state support mode shifts to the normal operation state support mode, the second rocker shaft and the connecting rod must be accommodated in the rocker arm support member. That is, a plurality of members must enter and leave the rocker arm support member when switching the support modes. Since switching easily fails, the reliability of the operation decreases.
The second reason is that the connecting rod is relatively thin. That is, the connecting rod may collide against the rocker arm support member and break when the support modes are switched. This decreases the reliability of the operation.
To reliably switch the rocker arm support modes, it may be possible to omit the first rocker shaft and the connecting rod of the above-described rocker shaft assembly and use only the second rocker shaft. In this case, the second rocker shaft is pulled out from the rocker arm and accommodated in the rocker arm support member in the cylinder resting state.
If this arrangement is used, however, the swinging center of the rocker arm moves from a correct position in the cylinder resting state so the second rocker shaft cannot fit into the shaft hole of the rocker arm support member when the cylinder resting state support mode shifts to the normal operation state support mode.